Add FuseConcatPass to eliminate redundant concat ops (#18827)

[ryan-monroe]

Summary:

Concat (torch.cat) in the Gen2 Executorch ARM/Ethos-U stack is lowered to
TOSA CONCAT, which Vela then converts to N x MemoryCopy operations — real
DMA data movement on the NPU. This pass eliminates concat operations that
can be proven unnecessary at the FX graph level, preventing Vela from
generating MemoryCopy ops entirely.

Inspired by Espresso's concat elimination techniques
(bolt/nn/espresso/transforms/remove_nops.py), three patterns are handled:

1. Single-input concat: cat([x]) is a no-op, replaced with x.
2. Concat-then-slice: if every consumer of cat([a, b, ...]) is a
   slice_copy that extracts exactly one original input, bypass both.
3. Slice-then-concat: if contiguous slices of the same tensor are
   concatenated back, the result is the original tensor.

Differential Revision: D97667069

[pytorch-bot]

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• [CI[B200] Smoke test encounters CUDA Unknown error for dgxb200-03 and dgxb200-04

❌ 1 New Failure, 5 Pending, 2 Unrelated Failures

As of commit 4056395 with merge base 875f7c8 ():

NEW FAILURE - The following job has failed:

• Test ARM Backend / test-arm / test-backend-linux (arm_vgf_fp, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t b0533266074755ec4bf3e93cb109ab7ee4ef3cf00983a1e8c2d9b12043c281f9 /exec failed with exit code 92

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@ryan-monroe has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97667069.

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[ryan-monroe]

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